The present invention relates to waterborne coating compositions and to methods for the preparation of multi-layered coated articles utilizing said compositions and to the resultant coated articles.
In the coating of substrates such as, for example automobiles, where a coating is desired which provides a lustrous appearance, it has been well appreciated to provide the substrate with several coating layers in order to achieve the desired effect. Therefore, typically a pigmented coating composition is first applied to the substrate followed by a transparent topcoat. By transparent is meant a clear coating, that is one which does not contain pigmentation or contains only transparent pigments. Such a coating system is commonly referred to as "clear-over-color" or "color plus clear".
Automotive coatings containing metallic pigments such as aluminum flake are generally utilized to obtain the glossy lustrous appearance which is characteristically sought. In order to achieve the preferred appearance of such metallic coating compositions it is very important that the metallic pigment orient such that it is parallel to the coated surface. The alignment of the pigment in this fashion provides for the most desirable appearance, especially with respect to the "flop" of the coating. By "flop" is meant the visual change in brightness or lightness of the metallic aluminum flake with a change in viewing angle, that is, a change from 90 to 180 degrees. The greater the visual change from light to dark appearance, the better the flop. The flop accentuates the lines and curves of an automobile; therefore, it is very important in achieving the sought after appearance of the coating. It is also very important that the metallic pigment be uniformly oriented across the surface of the substrate, otherwise blotchy areas of light and dark color will be evident. This condition is commonly known as mottling.
Over the past several years the trend in the automotive industry has been to reduce atmospheric pollution caused by the volatile solvents which are emitted during the painting process. One approach to emissions control has been the use of waterborne coating compositions as the pigmented color coat in the "color plus clear" system.
Waterborne coating compositions, however, are not without attendant disadvantages. For example, such coatings often have a narrow application window in which excellent film properties are obtained. That is, it is difficult to obtain smooth films, free of solvent popping over a wide range of relative humidities. In addition, at high humidities, mottling of the film is frequently observed.
It is desirable, therefore, to have waterborne coating compositions which have excellent resistance to mottling and popping, have good flop, and also have reduced sensitivity to fluctuations in relative humidity.